Cardiac dysrhythmias have been seen in patients receiving immunotherapy (IM) for various cancers. The types and frequencies of these dysrhythmias are not well documented. This prospective study documented the incidence of dysrythmias of 20 lM cancer patients. Holter monitoring was performed prior to lM (x=21 .3 hrs) and intra IM (x=45.3 hrs). Paired t-tests compared differences in incidences of supraventricular, ventricular premature beats/1000 QRS complexes (SVPB/1000 and VPB/1000) and bradycardic events respectively. Results obtained were: SVPB/1000 pre-IM (x=1.41), intra-IM (x=.77): (t=.915, p=.37). VPB/1000 pre=IM (x=2.46), intra-IM (x=3.71); (t=.71, p=.49). Bradycardic events pre-IM (x=6), intra- IM (x=<0); (t-1.64. p=.12). These results indicate that there were no changes in dysrhythmias post IM initiation, illustrating the importance of obtaining baseline ECG data to avoid overestimation of dysrythmias attributed to IM.